Inverter ballast circuit for fluorescent lamp

ABSTRACT

An oscillator including a single transistor is connected in circuit with the primary winding of a stepup transformer. The terminals of a fluorescent lamp are connected between the primary and secondary of the transformer. One filament of the lamp is connected in the base circuit of the transistor. The filament acts as a constant current device to limit the current flowing in the base circuit and thereby permit the inverter to operate over a wide range of input voltages.

United States Patent [191 May [451 May 6,1975

[ INVERTER BALLAST CIRCUIT FOR FLUORESCENT LAMP [75] Inventor: RandallL. May, Andover, Kans.

[73] Assignee: The Coleman Company, Wichita,

Kans.

[22] Filed: July 23, 1973 [21] Appl. No.: 381,858

[52] US. Cl. 315/101; 315/105; 315/206;

315/221; 315/DIG. 5; 3l5/DIG. 7 [51] Int. Cl H051) 41/29; HOSb 41/392[58] Field of Search 315/94-98,

315/101, 105, 200, 206, 221, 278, DIG. 2, DIG. 5, DIG. 7, 177-179, 181;331/112 [56] References Cited UNITED STATES PATENTS 3,432,723 3/1969Miller et al. 315/94 3,629,648 12/1971 Brown et a]. 315/98 PrimaryExaminer-R. V. Rolinec Assistant ExaminerE. R. LaRoche Attorney, Agent,or Firm-Dawson, Tilton, Fallon & Lungmus [5 7] ABSTRACT An oscillatorincluding a single transistor is connected in circuit with the primarywinding of a stepup transformer. The terminals of a fluorescent lamp areconnected between the primary and secondary of the transformer. Onefilament of the lamp is connected in the base circuit of the transistor.The filament acts as a constant current device to limit the currentflowing in the base circuit and thereby permit the inverter to operateover a wide range of input voltages.

2 Claims, 1 Drawing Figure INVERTER BALLAST CIRCUIT FOR FLUORESCENT LAMPBACKGROUND AND SUMMARY The present invention relates to an inverterballast circuit for fluorescent lamps. There are many such circuitsalready known. and the purpose of these circuits is to convert a DC.voltage to an alternating voltage of suitable magnitude andifrequency toenergize a fluores cent lamp.

One of the problems with prior inverter ballast circuit designs is thatthey donot operate well over a wide range of input D.C. voltages. One ofthe major uses of inverter ballast circuits is in recreational vehicles;and depending upon the vehicle design and the state or condition of thebatteries of the vehicles, an inverter ballast circuit, to be practical,must operate with a range of input voltages extending from 8 or 9 voltsup to 25 volts.

The present invention has as a principal object, the design of aninverter ballast circuit for operating fluorescent lamps which iscapable of operation over a wide range of input D.C. voltages. Thepresent circuit includes an oscillator having a single transistor as anactive element. The transistor is connected in circuit with the primarywinding of a step-up transformer. The terminals of a fluorescent lampare connected between the primary and secondary windings of thetransformer.

One filament of the lamp is connected in the base circuit of thetransistor. preferably in series between the base terminal of thetransistor and one terminal of the primary winding of the transformer.The filament thus acts as a constant current device to limit the gain ofthe transistor independent of the input voltage source. That is to say.with the filament of the lamp in series with the base circuit of thetransistor. if the input voltage is increased. a slight additionalaverage current through the filament will cause it to heat up. therebyincreasing its resistance dramatically. and tending to stabilize thebase current at a fixed value. Similarly, if the input voltage isdecreased, a corresponding slight decrease in the base current willsubstantially reduce the resistance of the heater filament of the lamp,and thereby permit more base current to flow than would otherwise flowif the resistance of the heater filament were not so temperaturedependent.

The light given off by a fluorescent lamp is a linear function of theapplied voltage, and for slight changes in applied voltage, the changein light intensity is not very noticeable. Hence, the present inventionprovides a suitable inverter ballast circuit for fluorescent lamps overa wide range of input voltages.

Another feature of the present invention is that it produces a betterstarting circuit, especially in cold weather. The reason for this isthat when the circuit is first turned on, the filament of the tube iscold. thereby providing a maximum of base current and. hence. a maximumvoltage output from the transformer to start the lamp. The filamentheats rapidly with a maximum of base current. and this has an advantagein that a tube with a hot filament starts easier than a tube with coldfilaments.

Another advantage of the present invention, and one that is consideredquite important, is that if the fluorescent lamp is dislodged from itssocket, the circuit is shut off--that is, completely prevented fromoperating-- -because the base circuit of the transistor is interrupted.In many prior circuits, if the fluorescent lamp fell out of its socket,not only would the inverter circuit continue to operate. but it wouldgenerate an excessive voltage because such circuits were designed toproduce a high voltage across the tube terminals until the lamp was lit.and the voltage would then drop as the lamp began to conduct. If thelamp did not conduct or became dislodged, the circuits continued togenerate an excessively high voltage. Under these circumstances, thecircuit also continues to draw excess current which will destroy thetransistor and associated circuitry on the inverter.

The present invention also has the advantage that one or morefluorescent lamps can be connected in series with each other and excitedby the same inverter ballast circuit. In such an arrangement. a No. Ilamp would have its filament connected in series with the base circuitof the transistor in the oscillator. Thus. if any single tube goes out,it is simply replaced; but if all of the lamps go out. the No. l lamp isreplaced because it is known that if the No. l lamp burns out. it willinhibit operation of the other lamps because the inverter ballastcircuit will not be capable of operating.

Other features and advantages of the present invention wil be apparentto persons skilled in the art from the following detailed description ofa preferred embodiment accompanied by the drawing.

THE DRAWING The single FIGURE is a circuit schematic diagram of aninverter ballast circuit incorporating the present invention.

DETAILED DESCRIPTION Referring to the drawing. reference numeral 10generally designates a fluorescent lamp having a pair of terminals 11 atone end and a similar pair of terminals 12 at the other end. A filamentI3 is connected between the terminals 11, and a similar filament I4 isconnected between the terminals 12 is not used in the illustratedembodiment.

A source of input DC. voltage. such as a storage battery. has itspositive terminal connected to an input terminal 16 of the circuit, andits negative terminal connected to the circuit terminal 17 which may beground.

The terminal 16 is connected by means ofa diode 18 to the collector ofatransistor generally designated 19. A capacitor is connected from thecollector of the transistor 19 to ground.

The emitter of transistor 19 is connected to a terminal 21 of atransformer T which has a primary winding generally designated 22 and asecondary winding 23. The primary winding 22 includes a first series ofturns 24 and a second series of turns 25 connected in series. Theterminal 21. previously mentioned, consists of a tap between thewindings 24, 25.

A capacitor 27 is connected across the entire primary 22, to suppresstransients, and the lamp 10 is connected between the primary andsecondary windings, in series with a current limiting capacitor 28. Thatis. one of the terminals 11 is connected to the ungrounded terminal ofthe primary winding 22, and one of the terminals 12 is connected to theungrounded terminal of the secondary 23 via capacitor 28.

The other terminal 11 of the lamp I0 is connected in series with acapacitor 30 to the base terminal of transistor 19. A diode 31 isconnected in the polarity shown between the base and emitterterminals'of the transistor 19; and a leakage resistor 32 is connectedbetween the base and collector terminals of the transistor 19.

OPERATION With the lamp and circuit connected as shown in the drawing,and a source of DC. voltage connected to the terminals l6, 17 in thepolarity shown, the circuit operation will now be described. It will beappreciated that a manually operated switch is normally incorporatedinto the circuit for turning the lamp on and off, and such a switch maypreferably be located in series with the diode 18.

When power is applied to the transistor 19, it will be appreciated thatthe base is biased positively relatively to the emitter, and this willinitially cause the transistor to conduct. The resulting current flowsthrough the section 24 ofthe primary winding 22 of the transformer T.Thus, the terminal 21 will begin to go positive, as will thecorresponding dotted terminals of the section 25 of the primary 22 andthe secondary winding 23. When the dotted terminal of the section 25 ofthe primary 22 goes positive, the transistor 19 becomes more forwardbiased, thereby resulting in additional emitter current, the final valueof which will depend upon the base current which, in turn, is dependentupon the value of resistance in the filament l3. Initially, the filament13 is cold, so the initial current energizing transformer T is large,and the secondary voltage is also large, thereby supplying a largevoltage to start the tube 10.

Another way to view this operation is that when the filamenflli is cold,its resistance is low, so the base current is high, thereby reducing thevoltage across the collector-emitter terminals of the transistor 19.Since the input voltage is constant, the remaining voltage appearsdirectly across the section 24 of the primary winding 22.

As the" current flowing in the secondary changes slopepolarity, thevoltage across the primary section 25 and secondary 23 will alsodiminish, and the reduced voltage at the primary section 25 will reducethe base corrent in transistor 19. The oscillations in the secondarywill thus drive the transistor, but the magnitude of base currentremains substantially constant.

As the temperature of the filament 13 increases, its resistanceincreases dramatically, thereby reducing the base current until astable, equilibrium position is reached.

The operation thus far described has been only for the starting andcontinuous operation of the circuit for a given voltage source. One ofthe principal features of the present invention is that if the voltagesource changes in magnitude, or if the circuit is used with a differentvoltage source of different magnitude, the ballast circuit readilyaccommodates itself to the change in input voltage magnitude whilesupplying the lamp 10 with substantially the same terminal voltage foroperation.

For example, if the input voltage across the terminals l6, 17 isreduced, the average current flowing in the filament 13 iscorrespondingly reduced and this will cause the filament to coolsomewhat, thereby decreasing its resistance in a nonlinear fashion sothat the resulting decrease in base current is almost negligible. lnother words, I have found that the filament of the fluorescent tube, inthe circuit illustrated, acts as a substantially constant currentsource. If the voltage across the terminal l6, 17 increases, theresulting base current increase will raise the temperature of thefilament 13, thereby dramatically increasing its resistance value, andpermitting only a negligible increase in base current.

It will also be observed that if the tube 10 becomes dislodged from thecircuit, the transistor 19 cannot oscillate because the base circuit isdisconnected from the regenerating section 25 of the primary 22 of thetransformer T. That is to say. there is nothing to turn the base circuitoff and cause it to oscillate.

In the embodiment shown, the ratio of the turns of the windings 22, 25,23 is as follows: 13/9/250. This turns ratio is useful in energizing asingle lamp. When it is desired to use more than one lamp, they may beconnected in series and the voltages adjusted accordingly. When such anarrangement is used, only the No. 1 tube has its filament connected inseries with the base of the transistor 19, and the remaining tubes areconnected in series. Thus, if any of the tubes other than the No. 1 tubeburns out, it is simply replaced, but if the No. 1 tube is burned out,this fact will be made known because none of the tubes will then burn.

Having thus described in detail a preferred embodiment of the presentinvention, persons skilled in the art will be able to modify certain ofthe structure which has been illustrated and to substitute equivalentelements for those disclosed while continuing to practice the principleof the invention; and it is, therefore, intended that all suchmodifications and substitutions be covered as they are embraced withinthe spirit and scope of the appended claims.

We claim:

1. An inverter ballast circuit for energizing a fluorescent lamp from asource of direct voltage comprising: a transistor having a collector, anemitter and a base terminal, said collector being connected to oneterminal of said source; a transformer having a primary winding and asecondary winding, said primary winding including an input section and afeedback section; means for connecting said lamp between said primaryand secondary winding; means for connecting said feedback windingsection of said transformer in series with a filament of said lamp andwith the base-emitter junction of said transistor, the emitter junctionof said transistor being further connected in circuit with said inputwinding section of said primary winding of said transformer such thatcurrent flowing through said transistor will energize said input windingsection to induce a voltage in said secondary and in said feedbackwinding section, said input winding and said transistor being arrangedin an oscillatory circuit, whereby as the current in said input windingsection increases, said filament will limit the input current to saidtransistor, said filament being characterized as being a substantiallyconstant current element over its operating range, thereby to controlthe output voltage of said transistor.

2. In an inverter ballast circuit for energizing a fluorescent lamp froma source of direct voltage with an oscillating circuit having an activeelement and controlled by an input current, the improvement comprising:transformer means including a primary winding and a secondary winding,said primary winding including an input winding section and a feedbackwinding.

section; means connectingsaid lamp between said secondary and saidprimary windings; and means connect-.

ing said feedback winding section, a filament of said LII keep the inputcurrent to said active element substantially constant whereby asubstantially constant voltage is applied and energizes said lampindependent of the magnitude of said voltage source over a given range.

UMTED STATES PATENT OFFICE QERTHEATE 9F CURRECTION Patent No. 3 882 354InVentor(s -1' L May Dated May 6, 1975 It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

assignees name to:

The Coleman Company, Inc.

[SEAL] Arrest:

RUTH C. MASON 'Arlc'sling Officer gigmd and fiealzd' this fif h Day ofAugust1975 C. MARSHALL DANN ('ununissinncr of Parents and Trarlwnurks

1. An inverter ballast circuit for energizing a fluorescent lamp from asource of direct voltage comprising: a transistor having a collector, anemitter and a base terminal, said collector being connected to oneterminal of said source; a transformer having a primary winding and asecondary winding, said primary winding including an input section and afeedback section; means for connecting said lamp between said primaryand secondary winding; means for connecting said feedback windingsection of said transformer in series with a filament of said lamp andwith the base-emitter junction of said transistor, the emitter junctionof said transistor being further connected in circuit with said inputwinding section of said primary winding of said transformer such thatcurrent flowing through said transistor will energize said input windingsection to induce a voltage in said secondary and in said feedbackwinding section, said input winding and said transistor being arrangedin an oscillatory circuit, whereby as the current in said input windingsection increases, said filament will limit the input current to saidtransistor, said filament being characterized as being a substantiallyconstant current element over its operating range, thereby to controlthe output voltage of said transistor.
 2. In an inverter ballast circuitfor energizing a fluorescent lamp from a source of direct voltage withan oscillating circuit having an active element and controlled by aninput current, the improvement comprising: transformer means including aprimary winding and a secondary winding, said primary winding includingan input winding section and a feedback winding section; meansconnecting said lamp between said secondary and said primary windings;and means connecting said feedback winding section, a filament of saidlamp and said active element in circuit such that the input currentcontrolling said active element passes from said feedback windingthrough said filament, whereby as the magnitude of said voltage sourceincreases, the current through said filament increases to substantiallyincrease the resistance thereof, thereby to keep the input current tosaid active element substantially constant whereby a substantiallyconstant voltage is applied and energizes said lamp independent of themagnitude of said voltage source over a given range.